fix(slack): Serialize blocks key for payload (#69021)

Json payload for slack request requires the blocks key to be serialized
as a string
Resolves:
https://github.com/getsentry/team-core-product-foundations/issues/188#issue-2192978404

Author: ykamo001

src/sentry/integrations/slack/service.py

@@ -13,6 +13,7 @@
 from sentry.models.rule import Rule
 from sentry.notifications.notifications.activity import EMAIL_CLASSES_BY_TYPE
 from sentry.types.integrations import ExternalProviderEnum
+from sentry.utils import json
 
 _default_logger = getLogger(__name__)
 
@@ -166,6 +167,8 @@ def _handle_parent_notification(
             block, fallback_text=notification_to_send
         )
         payload.update(slack_payload)
+        # TODO (Yash): Users should not have to remember to do this, interface should handle serializing the field
+        payload["blocks"] = json.dumps(payload.get("blocks"))
         try:
             client.post("/chat.postMessage", data=payload, timeout=5)
         except Exception as err: